Interpretive Summary: Biological control of agricultural weeds using soil microorganisms has remained an elusive goal for decades. One knowledge gap that needs to be addressed in order to have support for further investigation of this management tactic is whether soil microbes and weeds can form preferential, lasting relationships in a highly disturbed soil environment, as is found in agricultural fields. Using soils from five states in the north central U.S., we performed a greenhouse study of microbial affinity for, and effect on, two agricultural weeds, giant ragweed and common sunflower. Affinity-effect relationships were stronger for ragweed than for sunflower, and ragweed relationships showed consistent negative effects. Ragweed plant-soil feedback involved multiple microbial species, resulting in strong, consistent affinity-effect relationships in spite of large-scale microbial variability between trials. In contrast, sunflower plant-soil feedback may involve just a few key players, making it more sensitive to underlying microbial variation. The outlook for biocontrol of giant ragweed using soil microbes is therefore stronger than for common sunflower.

Technical Abstract: Plant-soil feedback involving soil microorganisms can regulate plant populations. To participate in plant-soil feedback, microorganisms must display an affinity for plant species, and they must produce consistent effects on plant growth. We tested the validity and strength of microbial affinity-effect relationships using multiple “home-and-away” experiments involving giant ragweed (Ambrosia trifida L.) and common sunflower (Helianthus annuus L.). DNA fingerprints were used to characterize microbial communities in these experiments. We used canonical ordination and partial least squares regression to develop indices expressing each microorganism’s affinity for ragweed or sunflower and its effect on plant biomass. We used linear regression to analyze the relationship between microbial affinity and effect. Significant linear affinity-effect relationships were found in 75% of cases. Affinity-effect relationships were stronger for ragweed than for sunflower, and ragweed relationships showed consistent negative feedback. Ragweed plant-soil feedback involved multiple microbial taxa, resulting in strong, consistent affinity-effect relationships in spite of large-scale microbial variability between trials. In contrast, sunflower plant-soil feedback may involve just a few key players, making it more sensitive to underlying microbial variation. We propose that affinity-effect relationship can be used to determine key microbial players in plant-soil feedback against a low “signal-to-noise” background of complex microbial datasets.